A. Field of the Invention
This invention relates generally to the field of devices used to measure blood pressure. More particularly, the invention relates to method for continuously obtaining blood pressure data, and related information such as pulse pressure, pulse rate and arterial compliance, from a patient using a non-invasive optical sensor.
B. Statement of Related Art
Non-invasive systems for continuous monitoring of blood pressure, for example during anesthesia, currently exist. Representative patents include the patents to Shinoda et al., U.S. Pat. No. 5,165,416; the patents to Erkele et al., U.S. Pat. No. 4,802,488 and 4,799,491; Jones et al., U.S. Pat. No. 5,140,990, Jackson et al., U.S. Pat. No. 5,485,848 and Pytel et al., U.S. Pat. No. 5,195,522. It is also known to use optical sensors as the means to acquire blood pressure data. See the patents to Butterfield, et al., U.S. Patent 5,908,027; 5,158,091; 5,261,412 and 5,273,046; Cerwin, U.S. Pat. No. 5,984,874 and Tenerz et al., U.S. Pat. No. 5,018,529. The above-referenced patents are incorporated by reference herein.
Prior art mechanical sensors commonly measure blood pressure by detecting transducer changes that are proportional to the detected changes in external force measured at the skin surface during pulsation. These sensors depend on mechanical parts and are therefore more subject to breakdown due to moving parts, and are larger in size thus requiring more space for fitting it on the patient skin. They are typically large in actual size. These sensors employ the use of a single sensor, or an array of sensors from which only one (the one with the highest signal strength) is selected for measurement. Such sensors only cover a small surface area on the skin and are therefore very sensitive to initial exact placement of the sensor on top of the artery. They are also sensitive to movement or minor accidental repositioning. This typically invalidates all calibrations, requiring a need for re-calibrating the system with an air cuff pressure reference. Providing a corrective feedback mechanism for compensating for minor positional changes in sensor placement is not possible due to dependency on a single-point or single-sensor measurement. Furthermore, the resolution of these sensors to blood pressure changes at low level signal strength is not sufficient to obtain accurate results. Other sensors typically require higher hold down pressure (HDP) values in order to obtain a stronger signal due to their low sensitivity. They also offer no corrective feedback mechanism for compensating for minor variations in the hold down pressure, often requiring a need for re-calibration of the sensor at the new hold down pressure value.
Portable oscillometric wrist mounted blood pressure devices also exist, such as the Omron model HEM-609, but these are not intended for continuous blood pressure monitoring. The oscillometric method requires the patient to be at a rested state, and a cuff pressure to be applied by the device that is above the systolic blood pressure of the patient (thus temporarily cutting off circulation in the artery and causing discomfort).
Spacelabs"" Modular Digital Telemetry system offers an ambulatory blood pressure (ABP) option for wireless transmission of noninvasive blood pressure data to a central computer, however it is not a tonometric optical blood pressure monitor and it is transmit only.
The above-referenced ""027 Butterfield et al. patent describes a device and technique for measuring tonometric blood pressure non-invasively using a one-dimensional optical sensor array. The sensor used in the ""027 patent is also described in U.S. Pat. No. 5,158,091 to Butterfield et al. The array detects photo-radiation that is reflected off of a semiconductor, thermally sensitive diaphragm, with the diaphragm deflected in response to arterial pulsation. The diaphragm""s thermal properties affect how its surface is deflected. Such thermal properties are associated with calibration coefficients which are used for mapping measured deflections into mmHg blood pressure values. The calibration procedure requires taking such thermal properties into consideration, including a) thermal heating of the diaphragm, b) calibration for optimum vs. non-optimum applanation state of the underlying artery, and c) deformable and a nondeformable portions of the diaphragm so that calibration coefficients can be obtained to map measured sensor output signal into blood pressure.
The present invention is believed to be a substantial improvement over the type of sensors proposed in the prior art. The sensor itself does not depend on thermal considerations. The diaphragm or reflective surface in the present sensor is responsive to any input stress on its surface. Furthermore, a priori knowledge of the exact applanation state is not needed for proper calibration.
Additionally, the sensor is calibrated against a standard conventional air cuff for measuring blood pressure. The calibration procedure automatically compensates for variability that is inherent in patient anatomy and physiological parameters such as body weight, size, skin thickness, arterial depth, arterial wall rigidity and compliance, body fat, etc.. When the sensor is calibrated against known blood pressure (such as using an air-cuff system) all such detailed variables are individually and collectively integrated and linearized in the process of calibrating the sensor. In other words our calibration process is customized to the individual patient anatomy. Accordingly, the sensor and method of the invention produces more accurate results.
The ""027 patent describes a set of detectors which are arranged in a single dimensional row. Image processing techniques are not particularly applicable in the format of arrangement of the detectors. In contrast, the sensor and method of the present invention uses a two-dimensional array of photo-sensitve elements which is cabable of producing a digitized two-dimensional image of the underlying skin surface variations due to pulsation. The number and density of elements are significantly higher. Accordingly, the array produces an image that can be processed using image processing techniques, including image transformation algorithms to detect translation or rotation of the sensor. Image processing methods can also be used for filtering, calibrating, tracking, and error-correcting the output of the sensor.
The ""027 patent requires a mechanical assembly to provide a means for mechanically pushing the sensor onto the surface of skin tissue, and adjusting the force used for obtaining optimal artery applanation. The present invention does not require the need for such stress-sensing mechanical assembly for proper positioning and adjustment to achieve optimum applanation of the artery. The sensor does require a measurable hold down pressure to be applied on the sensor to produce measurable results for calibration purposes. The hold down pressure can be produced by mounting the sensor to a wrist watch band for example. Furthermore, the sensor and inventive method provide for compensating for changes in the hold down pressure between initial or calibration values of hold down pressure and values of hold down pressure later on when blood pressure data is obtained.
In a first aspect, a method is provided for obtaining blood pressure data from a patient using an optical blood pressure sensor placed against a patient""s body. The sensor includes a two-dimensional array of photo-sensitive elements that obtain image data of the surface of the patient""s body. Specifically, the array generates images of the deflection of the patient""s body due to arterial blood flow, such as by detection of photo-radiation (i.e., light) reflecting off a flexible reflective surface placed against the patient""s body. The scattering patterns are recorded as two-dimensional images. The images are in turn digitized and processed in accordance with the method of the invention.
The method includes a first step of calibrating the optical sensor. A first digitized two-dimensional calibration image of a portion of the patient""s body is obtained by the optical sensor, such as the patient""s wrist area in the vicinity of the radial artery. While the image is obtained, a blood pressure measurement is made of the patient, such as by using a conventional air-cuff sphygmomanometer. The blood pressure measurement is compared to at least one portion of the first image, such as one photo-sensitive element, or a group of such elements, to thereby obtain a calibration relationship between the selected portion of the first image (i.e., the digitized output signal for photo-sensitive elements corresponding to the selected portion of the image) and the blood pressure measurement. Preferably, a multitude of calibration images are obtained in both systolic and diastolic events, and the comparison between output signal and blood pressure measurement is performed for the set of images. A best fit linear polynomial relationship is found between blood pressure and output signal to thereby arrive at a more accurate calibration relationship.
With the sensor thus calibrated, it is now ready to be used to obtain blood pressure data from the patient. A second digitized two-dimension image of the selected portion of the patient""s body is obtained during a period in which the blood pressure data is sought from the patient. The calibration relationship that was derived for the selected portion of the first image (group of one or more photo-sensitive elements) is then applied to a corresponding portion of the second image, namely the set of selected photo-sensitive element or elements. Blood pressure data is thus derived from the application of the calibration relationship to the corresponding portion of the second image. If the blood pressure is the same, the digitized output signal for the selected portion of the calibration and second images would be expected to be the same, and the sensor would therefore report blood pressure data as being the same. If the output signal is different for the second image, linear scaling as provided by the calibration relationship is performed. The blood pressure data is thus derived from the scaled calibration relationship applied to the selected portion of the second image.
The selected portion of the calibration image(s), in the preferred embodiment, comprises a contour or set of locations having substantially the same image intensity values, and the calibration relationship is obtained for the contour. Alternatively, the selected portion of the calibration image could be a single location in said image, that is, a single photo-detector. The calibration relationship is obtained for the single photo-detector. The calibration relationship obtained for the single photo-detector is then applied to the same photo-detector""s output in the second image. Alternatively, the selected portion of the calibration image could consist of a set of locations in the first image (i.e., a set of photo-detectors) having substantially different image intensity values. The calibration relationship is obtained for this set of locations and applied to output signals from the set of photo-detectors from the second image and the results averaged to obtain blood pressure data.
The invention also contemplates the ability to compensate for changes in hold down pressure that is applied between the optical sensor and the patient, as such changes could affect the images generated by the array. Thus, the method may further comprise the step of measuring a first hold-down pressure being applied during calibration, measuring a second hold down pressure during the obtaining of the second image, and comparing the first hold down pressure with the second hold down pressure. If the second hold down pressure is substantially different from the first hold down pressure, an error message could be displayed to the user indicating that the sensor cannot obtain valid blood pressure data. If the differences are below a threshold level, a linear scaling may be performed for the blood pressure data (or the calibration relationship) in accordance with the difference between the first and second hold down pressures to arrive at an accurate blood pressure reading. In a preferred embodiment, the hold down pressure measurements are obtained with a strain-gauge type sensor formed as a two-dimensional, flexible membrane or surface that is built into the optical sensor and positioned immediately adjacent to the surface of the patient""s body.
In a preferred embodiment, the invention also preferably provides for the ability to compensate for rotation or translation of the optical sensor relative to the patient occurring between the time the calibration image is obtained, and when the second image is obtained. The rotation or translation of the optical sensor can be performed by application of correlation algorithms or other known image analysis techniques to the images generated by the array.
The sensor and inventive method is well suited to an application in which continuous measurements of blood pressure is desired. Thus, a multitude of digitized two-dimensional images can be obtained from the array over a data collection period of time. The frequency at which the images can be generated is a matter of design choice, and will depend on such factors as the readout rate of the sensor, the sampling rate of the electronics, and other factors. The images could be obtained at a rate of say 10 or even 100 per second. The steps of applying the calibration relation to the selected portion of the images and derivation of blood pressure data could be performed for each of the multitude of images, resulting in a continuous stream of blood pressure data. Alternatively, the images could be obtained or processed in a gating window around the period of when the systolic and diastolic events are expected to occur.
The generation of a multitude of digitized two dimensional images enables may useful image processing techniques to be performed on the images. For example, good tracking between measured estimates of blood pressure and actual blood pressure can be achieved by applying a Kalman filter with a one-step predictor. The predicted values can be used to correct for estimation errors, which helps prevent accumulation of error residuals in the reported blood pressure data. As another example, a spatial Finite Impulse Response (FIR) filter can be defined with appropriate coefficients to enhance detection and elimination of motion artifacts and noise, with the FIR filter applied to the multitude of two-dimensional images. As another example, reduction of motion artifacts and noise in sensor output can be obtained by means of application of a one dimensional temporal low pass filter on the output of each individual detector, or a spatial filter that is applied on a group of detectors output, or a spatial and temporal filter applied on multiple detector outputs. Additionally, the output from the detectors can be gated by the heart rate such that computation of end-systolic and end-diastolic pressure values is only considered during a short time-window around the time frame of expected end-systolic and end-diastolic event occurrence. Such timing can be determined and tracked dynamically by means of a Kalman filter, or other simpler methods, as a pulse period can experience an increase or decrease due to tachycardia or bradycardia or general arrythmia. Such gating enables the method to overlook any motion artifacts that might exist in time windows outside the gating window.
The generation of multiple images also allows for other useful physiologic data to be obtained. Arterial compliance can be estimated from a rate of change of skin displacement, which is derived from sequential images. The pulse rate can be derived from sequential images over a measured interval of time. Because of the fact that the sensor detection field spans a full plane of skin area, and because the sensor has a grid of photo-detectors and not just a single sensor, a dynamic image of the movement of a pulse pressure wave in the artery can be constructed. From such a pulse wave, it is possible to extract information such as blood flow rate, which can be measured as the pulse moves across the field of view of the sensor, crossing a known distance in a specific interval of time. Known distance can be determined by known separation between centers of photo-detectors in a grid of a particular detector density and size. The pulse could travel in any direction in the field of view, and the speed of which can be measured independent of its direction. Blood flow rate is then represented as the velocity at which systolic and diastolic events are marked at different points in the sensor field of view.
In another aspect, a method for processing output signals from a two-dimensional array of photo-sensitive elements to generate blood pressure data is provided. The two-dimensional array of photo-sensitive elements is incorporated into an optical blood pressure sensor adapted to be placed on the surface of a patient and obtain optical information as to movement of the patient""s skin in response to blood flow. The method comprises the steps of: generating a calibration relationship between output signals from the photo-sensitive elements to known blood pressure measurements, the calibration relationship associated with one or more photo-sensitive elements in the array. Two-dimensional images of the surface of a patient""s body are acquired during a period in which blood pressure information is sought for the patient. The images are digitized to thereby obtain a two-dimensional array of digital output values. The calibration relationship is applied to at least a portion of the array of digital output values to thereby derive the blood pressure data.
The methods of the present invention can be used in a variety of sensor designs. A presently preferred sensor assembly is described at length in this document. The sensor includes a housing adapted to be placed adjacent to the patient body, such as at the wrist, and a strap or similar means for applying a hold down force for the sensor in a location where blood pressure data is to be acquired during use of the sensor assembly. The sensor also includes a source of photo-radiation, which in preferred embodiment takes the form of one or more coherent light sources, such as laser diodes. The laser diodes may be arranged in a two dimensional array in one possible embodiment. The sensor also includes a two-dimensional, flexible reflective surface. The reflective surface may take the form of a reflective coating applied to a polymeric membrane. The reflective surface is nominally positioned relative to the radiation source such that the radiation travels in a direction normal to the reflective surface. The reflective surface is placed adjacent to the location on the patient where the blood pressure data is to be acquired, such as against the skin in the wrist area above the radial artery. A hold down pressure sensor, preferably in the form of a strain gauge arranged as a flexible membrane or diaphragm, is also incorporated into the sensor, and placed immediately in contact with the patient and adjacent to the reflective surface.
Radiation from the source is reflected off of the reflective surface onto a two-dimensional array of photo-detectors. The array of photo-detectors is nominally placed in the optical path of the radiation source, but they do not block all the radiation. Rather, they are spaced from one another sufficiently to allow incident radiation from the source to pass in between the detectors and impinge upon the reflective surface at an angle that is normal to the reflective surface. Systolic and diastolic blood pressure fluctuations in the patient are translated into deflections of the patient""s skin. These deflections cause corresponding deflections in the two dimensional reflective surface. The associated movement of said flexible reflective surface due to blood pulsation causes scattering patterns from the reflective surface to be detected by the two dimensional array of photo-detectors. After calibration as described herein, these scattering patterns, represented as digital values in a matrix of output values from the sensor as a whole, provide data from which blood pressure data can be extracted. In particular, a linear calibration relationship between blood pressure and output signal is applied to the matrix of output values, or, more typically, one or more of the entries in the matrix corresponding to a portion of the field of view selected for calibration and mapping.
These scattering patterns detected by the array of photo-detectors are processed either in a computing platform in the sensor assembly in accordance with the inventive methods, or alternatively in a remote processing unit such as a base unit. The optical sensor may communicate with the base unit using wireless transmission techniques, or the base unit may be connected to the optical sensor using convention wires or leads in a less preferred embodiment.
The methods of the present invention provide for a calibration relationship that is specific to the patient, and is therefore more accurate than prior art calibration techniques for optical sensors. The methods are completely noninvasive, and offer the ability to obtain blood pressure data and other physiologic data on a continuous basis. In an embodiment in which a wireless transmission technique is used for transmission of digitized image data to a remote base unit, the method offers improvements in patient mobility, convenience, flexibility, and the ability of the base unit to transfer real-time data and various statistical reports to a physician or log physiologic information in a data base for later review.